The intensity of the sky varies greatly over the day, and the primary cause of that intensity differs as well. When the sun is well above the horizon, direct scattering of sunlight (Rayleigh scattering) is the overwhelmingly dominant source of light. However, in twilight, the period of time between sunset and night and between night and sunrise, the situation is more complicated. Green flashes and green rays are optical phenomena that occur shortly after sunset or before sunrise, when a green spot is visible above the sun, usually for no more than a second or two, or it may resemble a green ray shooting up from the sunset point. Green flashes are a group of phenomena that stem from different causes, most of which occur when there is a temperature inversion (when the temperature increases with altitude rather than the normal decrease in temperature with altitude). Green flashes may be observed from any altitude (even from an aircraft). They are usually seen at an unobstructed horizon, such as over the ocean, but are also seen over cloud tops and mountain tops. Green flashes may also be observed at the horizon in association with the Moon and bright planets, including Venus and Jupiter.
The Earth’s shadow is the shadow that the Earth casts on its atmosphere. This atmospheric phenomenon is sometimes seen twice a day, around the times of sunset and sunrise. When the weather conditions and the observer’s viewing point permit a clear sight of the horizon, the shadow can be seen as a dark blue or greyish-blue band. Assuming the sky is clear, the Earth’s shadow is visible in the half of the sky opposite to the sunset or sunrise, and is seen as a dark blue band right above the horizon. A related phenomenon is the “Belt of Venus” or “anti-twilight arch”, a pink band that is visible above the dark blue band of the Earth’s shadow in the same part of the sky. There is no clear dividing line between the Earth’s shadow and the Belt of Venus: one colored band shades into the other in the sky.
Twilight is divided into three segments according to how far the sun is below the horizon, measured in segments of 6°. After sunset the civil twilight sets in; it ends when the sun drops more than 6° below the horizon. This is followed by the nautical twilight, when the sun is 6° and 12° below the horizon (heights of between −6° and −12°), after which comes the astronomical twilight, defined as the period from −12° to −18°. When the sun drops more than 18° below the horizon, the sky generally attains its minimum brightness.
Several sources can be identified as the source of the intrinsic brightness of the sky, namely airglow, indirect scattering of sunlight, scattering of starlight, and artificial light pollution.